In recent years, there has been a clear trend to downscale analytical instruments and to realize such instruments on the chip-based format. Examples can be found in every branch of analytical science, but this trend is especially important for gas analysis given the shear size of the field and the importance in science and engineering, industrial process control, improved environmental practices, aerospace applications, and other areas. Modern applications require the detection and identification of a wide variety of chemical species over vast concentration ranges. The analysis often must be carried out under limited conditions utilizing minimal resources.
For example, control of pure industrial processes, such as plasma processing, requires continuous monitoring of the process environment and the detection and identification of a wide range of chemical species. This monitoring often must be carried out in very limited conditions of chemically aggressive environments. Techniques that utilize miniature, rugged, but sensitive, devices are a top priority in addressing these issues. Each process, however, presents its own unique set of analytical requirements. Analytical instrumentation possessing universal response and providing primary sample identification can minimize the number of required instruments and, therefore, is highly desired for industrial process control, as well as for other applications in modern science and technology.
The need for miniature detection technologies capable of sample identification is strong because such detection technologies can be fully integrated with a chip-based gas chromatograph forming a micro-total analysis system. The on-chip combination of a molecular-specific detector and a gas chromatograph would be suitable for many different applications and would provide the benefits of device downscaling: portability, low weight, low power consumption, fast analysis, radical reduction in carrier gas consumption, easy integration, and low cost of production.
Advances in analytical science have produced sensitive, but small, devices capable of detecting gases. Chemical sensors are examples of such devices. These sensors, although tiny and highly sensitive with a wide response range, do not provide sample identification information. Only a few analytical techniques, such as Mass Spectrometry (MS), Ion Mobility Spectrometry (IMS), and Optical Emission Spectrometry (OES), coupled with Gas Chromatography (GC/MS, GC/IMS, and GC/OES), provide independent identification of chemical species. Portable mass spectrometers are characterized by high detection limits and resolution and are capable of offering the speed required for fast and ultra-fast gas chromatography. However, these units are technically complex, heavy and bulky, and require vacuum and high voltage for operation.
With the advance of micro-plasma sources, OES, specifically molecular emission spectroscopy, has received significant attention in recent years as a technology than is applicable to in-field gas analysis. As with MS, this technology involves resolving a task of miniaturization of the optical spectrum detection system to on-chip standards that is similar in complexity to that of MS.
IMS continues to be considered a promising technique for on-chip integration because it does not require optics as does OES or high-vacuum equipment as does MS. Presently, however, portable IMS devices used as in-field monitors experience a number of problems related mostly to poor selectivity and memory effects. In addition, IMS instruments require significant amounts of carrier gas and require a source of ionization that complicates the design.
Penning Ionization Electron Spectroscopy in plasma (PIES) also has been utilized in analytical science. PIES provides direct molecular identification of gas mixture components. A PIES detector does not simply collect electrons produced by Penning ionization, as is the case with the non-selective Metastable Ionization Detector and other helium ionization type detectors such as IMS, but relies on the measurement, in a discharge afterglow plasma, of the energy electrons formed by Penning ionization. The electron energy spectrum produced contains peaks specific for each component of the sample gas. Because the ionization energy is specific to each molecular species, the resultant data can be used to directly identify each component. This PIES electron energy spectrum is developed by means of a single collector electrode placed in the glow discharge plasma.
To date, the analysis of chemical species by PIES analytical technology has been for stagnant gas in a sealed discharge cell. The prior art known to applicant is devoid of teachings or suggestions that PIES analytical technology can be applied to detect and identify the presence of one or more gases in a flowing gaseous fluid.